Matching Items (70)
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This dissertation is on the study of structural and optical properties of some III-V and II-VI compound semiconductors. The first part of this dissertation is a study of the deformation mechanisms associated with nanoindentation and nanoscratching of InP, GaN, and ZnO crystals. The second part is an investigation of some

This dissertation is on the study of structural and optical properties of some III-V and II-VI compound semiconductors. The first part of this dissertation is a study of the deformation mechanisms associated with nanoindentation and nanoscratching of InP, GaN, and ZnO crystals. The second part is an investigation of some fundamental issues regarding compositional fluctuations and microstructure in GaInNAs and InAlN alloys. In the first part, the microstructure of (001) InP scratched in an atomic force microscope with a small diamond tip has been studied as a function of applied normal force and crystalline direction in order to understand at the nanometer scale the deformation mechanisms in the zinc-blende structure. TEM images show deeper dislocation propagation for scratches along <110> compared to <100>. High strain fields were observed in <100> scratches, indicating hardening due to locking of dislocations gliding on different slip planes. Reverse plastic flow have been observed in <110> scratches in the form of pop-up events that result from recovery of stored elastic strain. In a separate study, nanoindentation-induced plastic deformation has been studied in c-, a-, and m-plane ZnO single crystals and c-plane GaN respectively, to study the deformation mechanism in wurtzite hexagonal structures. TEM results reveal that the prime deformation mechanism is slip on basal planes and in some cases, on pyramidal planes, and strain built up along particular directions. No evidence of phase transformation or cracking was observed in both materials. CL imaging reveals quenching of near band-edge emission by dislocations. In the second part, compositional inhomogeneity in quaternary GaInNAs and ternary InAlN alloys has been studied using TEM. It is shown that exposure to antimony during growth of GaInNAs results in uniform chemical composition in the epilayer, as antimony suppresses the surface mobility of adatoms that otherwise leads to two-dimensional growth and elemental segregation. In a separate study, compositional instability is observed in lattice-matched InAlN films grown on GaN, for growth beyond a certain thickness. Beyond 200 nm of thickness, two sub-layers with different indium content are observed, the top one with lower indium content.
ContributorsHuang, Jingyi (Author) / Ponce, Fernando A. (Thesis advisor) / Carpenter, Ray W (Committee member) / Smith, David J. (Committee member) / Yu, Hongbin (Committee member) / Treacy, Michael Mj (Committee member) / Arizona State University (Publisher)
Created2013
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Description
High electron mobility transistors (HEMTs) based on Group III-nitride heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and electrical measurements, as well as other techniques. The dissertation was organized primarily into three topical areas: (1) characterization of near-gate defects in electrically stressed

High electron mobility transistors (HEMTs) based on Group III-nitride heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and electrical measurements, as well as other techniques. The dissertation was organized primarily into three topical areas: (1) characterization of near-gate defects in electrically stressed AlGaN/GaN HEMTs, (2) microstructural and chemical analysis of the gate/buffer interface of AlN/GaN HEMTs, and (3) studies of the impact of laser-liftoff processing on AlGaN/GaN HEMTs. The electrical performance of stressed AlGaN/GaN HEMTs was measured and the devices binned accordingly. Source- and drain-side degraded, undegraded, and unstressed devices were then prepared via focused-ion-beam milling for examination. Defects in the near-gate region were identified and their correlation to electrical measurements analyzed. Increased gate leakage after electrical stressing is typically attributed to "V"-shaped defects at the gate edge. However, strong evidence was found for gate metal diffusion into the barrier layer as another contributing factor. AlN/GaN HEMTs grown on sapphire substrates were found to have high electrical performance which is attributed to the AlN barrier layer, and robust ohmic and gate contact processes. TEM analysis identified oxidation at the gate metal/AlN buffer layer interface. This thin a-oxide gate insulator was further characterized by energy-dispersive x-ray spectroscopy and energy-filtered TEM. Attributed to this previously unidentified layer, high reverse gate bias up to −30 V was demonstrated and drain-induced gate leakage was suppressed to values of less than 10−6 A/mm. In addition, extrinsic gm and ft * LG were improved to the highest reported values for AlN/GaN HEMTs fabricated on sapphire substrates. Laser-liftoff (LLO) processing was used to separate the active layers from sapphire substrates for several GaN-based HEMT devices, including AlGaN/GaN and InAlN/GaN heterostructures. Warpage of the LLO samples resulted from relaxation of the as-grown strain and strain arising from dielectric and metal depositions, and this strain was quantified by both Newton's rings and Raman spectroscopy methods. TEM analysis demonstrated that the LLO processing produced no detrimental effects on the quality of the epitaxial layers. TEM micrographs showed no evidence of either damage to the ~2 μm GaN epilayer generated threading defects.
ContributorsJohnson, Michael R. (Author) / Mccartney, Martha R (Thesis advisor) / Smith, David J. (Committee member) / Goodnick, Stephen (Committee member) / Shumway, John (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2012
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Description
The theory of quantum electrodynamics predicts that beta decay of the neutron into a proton, electron, and anti-neutrino should be accompanied by a continuous spectrum of photons. A recent experiment, RDK I, reported the first detection of radiative decay photons from neutron beta decay with a branching ratio of (3.09

The theory of quantum electrodynamics predicts that beta decay of the neutron into a proton, electron, and anti-neutrino should be accompanied by a continuous spectrum of photons. A recent experiment, RDK I, reported the first detection of radiative decay photons from neutron beta decay with a branching ratio of (3.09 ± 0.32) × 10-3 in the energy range of 15 keV to 340 keV. This was achieved by prompt coincident detection of an electron and photon, in delayed coincidence with a proton. The photons were detected by using a single bar of bismuth germanate scintillating crystal coupled to an avalanche photodiode. This thesis deals with the follow-up experiment, RDK II, to measure the branching ratio at the level of approximately 1% and the energy spectrum at the level of a few percent. The most significant improvement of RDK II is the use of a photon detector with about an order of magnitude greater solid angle coverage than RDK I. In addition, the detectable energy range has been extended down to approximately 250 eV and up to the endpoint energy of 782 keV. This dissertation presents an overview of the apparatus, development of a new data analysis technique for radiative decay, and results for the ratio of electron-proton-photon coincident Repg to electron-proton coincident Rep events.
ContributorsO'Neill, Benjamin (Author) / Alarcon, Ricardo (Thesis advisor) / Drucker, Jeffery (Committee member) / Lebed, Richard (Committee member) / Comfort, Joseph (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2012
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Description
HgCdTe is currently the dominant material for infrared sensing and imaging, and is usually grown on lattice-matched bulk CdZnTe (CZT) substrates. There have been significant recent efforts to identify alternative substrates to CZT as well as alternative detector materials to HgCdTe. In this dissertation research, a wide range of transmission

HgCdTe is currently the dominant material for infrared sensing and imaging, and is usually grown on lattice-matched bulk CdZnTe (CZT) substrates. There have been significant recent efforts to identify alternative substrates to CZT as well as alternative detector materials to HgCdTe. In this dissertation research, a wide range of transmission electron microscopy (TEM) imaging and analytical techniques was used in the characterization of epitaxial HgCdTe and related materials and substrates for third generation IR detectors. ZnTe layers grown on Si substrates are considered to be promising candidates for lattice-matched, large-area, and low-cost composite substrates for deposition of II-VI and III-V compound semiconductors with lattice constants near 6.1 Å. After optimizing MBE growth conditions including substrate pretreatment prior to film growth, as well as nucleation and growth temperatures, thick ZnTe/Si films with high crystallinity, low defect density, and excellent surface morphology were achieved. Changes in the Zn/Te flux ratio used during growth were also investigated. Small-probe microanalysis confirmed that a small amount of As was present at the ZnTe/Si interface. A microstructural study of HgCdTe/CdTe/GaAs (211)B and CdTe/GaAs (211)B heterostructures grown using MBE was carried out. High quality MBE-grown CdTe on GaAs(211)B substrates was demonstrated to be a viable composite substrate platform for HgCdTe growth. In addition, analysis of interfacial misfit dislocations and residual strain showed that the CdTe/GaAs interface was fully relaxed. In the case of HgCdTe/CdTe/ GaAs(211)B, thin HgTe buffer layers between HgCdTe and CdTe were also investigated for improving the HgCdTe crystal quality. A set of ZnTe layers epitaxially grown on GaSb(211)B substrates using MBE was studied using high resolution X-ray diffraction (HRXRD) measurements and TEM characterization in order to investigate conditions for defect-free growth. HRXRD results gave critical thickness estimates between 350 nm and 375 nm, in good agreement with theoretical predictions. Moreover, TEM results confirmed that ZnTe layers with thicknesses of 350 nm had highly coherent interfaces and very low dislocation densities, unlike samples with the thicker ZnTe layers.
ContributorsKim, Jae Jin (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha R. (Committee member) / Alford, Terry L. (Committee member) / Crozier, Peter A. (Committee member) / Arizona State University (Publisher)
Created2012
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Description
The research described in this dissertation involved the use of transmission electron microscopy (TEM) to characterize II-VI and III-V compound semiconductor quantum dots (QDs) and dilute-nitride alloys grown by molecular beam epitaxy (MBE) and intended for photovoltaic applications. The morphology of CdTe QDs prepared by the post-annealing MBE method were

The research described in this dissertation involved the use of transmission electron microscopy (TEM) to characterize II-VI and III-V compound semiconductor quantum dots (QDs) and dilute-nitride alloys grown by molecular beam epitaxy (MBE) and intended for photovoltaic applications. The morphology of CdTe QDs prepared by the post-annealing MBE method were characterized by various microscopy techniques including high-resolution transmission electron microscopy (HR-TEM), and high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM). Extensive observations revealed that the of QD shapes were not well-defined, and the QD size and spatial distribution were not determined by the amount of CdTe deposition. These results indicated that the formation of II-VI QDs using a post-annealing treatment did not follow the conventional growth mechanism for III-V and IV-IV materials. The structural properties of dilute-nitride GaAsNx films grown using plasma-assisted MBE were characterized by TEM and HAADF-STEM. A significant amount of the nitrogen incorporated into the dilute nitride films was found to be interstitial, and that fluctuations in local nitrogen composition also occurred during growth. Post-growth partial relaxation of strain resulted in the formation of {110}-oriented microcracks in the sample with the largest substitutional nitrogen composition. Single- and multi-layered InAs QDs grown on GaAsSb/GaAs composite substrates were investigated using HR-TEM and HAADF-STEM. Correlation between the structural and optoelectronic properties revealed that the GaAsSb barrier layers had played an important role in tuning the energy-band alignments but without affecting the overall structural morphology. However, according to both XRD measurement and electron microscopy the densities of dislocations increased as the number of QD layers built up. An investigation of near-wetting layer-free InAs QDs incorporated with AlAs/GaAs spacer layers was carried out. The microscopy observations revealed that both embedded and non-embedded near-wetting layer-free InAs QDs did not have well-defined shapes unlike conventional InAs QDs. According to AFM analysis and plan-view TEM characterization, the InAs QDs incorporated with spacer layers had smaller dot density and more symmetrical larger sizes with an apparent bimodal size distribution (two distinct families of large and small dots) in comparison with conventional InAs QDs grown without any spacer layer.
ContributorsTang, Dinghao (Author) / Smith, David J. (Thesis advisor) / Crozier, Peter A. (Committee member) / Liu, Jingyue (Committee member) / Mccartney, Martha R (Committee member) / Arizona State University (Publisher)
Created2014
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ABSTRACT This thesis focuses on structural characterizations and optical properties of Si, Ge based semiconductor alloys. Two material systems are characterized: Si-based III-V/IV alloys, which represent a possible pathway to augment the optical performance of elemental silicon as a solar cell absorber layer, and Ge-based Ge1-ySny and Ge1-x-ySixSny systems which

ABSTRACT This thesis focuses on structural characterizations and optical properties of Si, Ge based semiconductor alloys. Two material systems are characterized: Si-based III-V/IV alloys, which represent a possible pathway to augment the optical performance of elemental silicon as a solar cell absorber layer, and Ge-based Ge1-ySny and Ge1-x-ySixSny systems which are applicable to long wavelength optoelectronics. Electron microscopy is the primary tool used to study structural properties. Electron Energy Loss spectroscopy (EELS), Ellipsometry, Photoluminescence and Raman Spectroscopy are combined to investigate electronic band structures and bonding properties. The experiments are closely coupled with structural and property modeling and theory. A series of III-V-IV alloys have been synthesized by the reaction of M(SiH3)3 (M = P, As) with Al atoms from a Knudsen cell. In the AlPSi3 system, bonding configurations and elemental distributions are characterized by scanning transmission electron microscopy (STEM)/EELS and correlated with bulk optical behavior. The incorporation of N was achieved by addition of N(SiH3)3 into the reaction mixture yielding [Al(As1-xNx)]ySi5-2yalloys. A critical point analysis of spectroscopic ellipsometry data reveals the existence of direct optical transitions at energies as low as 2.5 eV, well below the lowest direct absorption edge of Si at 3.3 eV. The compositional dependence of the lowest direct gap and indirect gap in Ge1-ySny alloys extracted from room temperature photoluminescence indicates a crossover concentration of yc =0.073, much lower than virtual crystal approximation but agrees well with large atomic supercells predictions. A series of Ge-rich Ge1-x-ySixSny samples with a fixed 3-4% Si content and progressively increasing Sn content in the 4-10% range are grown and characterized by electron microscopy and photoluminescence. The ternary represents an attractive alternative to Ge1-ySny for applications in IR optoelectronic technologies.
ContributorsJiang, Liying (Author) / Menéndez, Jose (Thesis advisor) / Kouvetakis, John (Thesis advisor) / Smith, David J. (Committee member) / Chizmeshya, Andrew V.G (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2014
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This dissertation presents research findings on the three materials systems: lateral Si nanowires (SiNW), In2Se3/Bi2Se3 heterostructures and graphene. The first part of the thesis was focused on the growth and characterization of lateral SiNW. Lateral here refers to wires growing along the plane of substrate; vertical NW on the other

This dissertation presents research findings on the three materials systems: lateral Si nanowires (SiNW), In2Se3/Bi2Se3 heterostructures and graphene. The first part of the thesis was focused on the growth and characterization of lateral SiNW. Lateral here refers to wires growing along the plane of substrate; vertical NW on the other hand grow out of the plane of substrate. It was found, using the Au-seeded vapor – liquid – solid technique, that epitaxial single-crystal SiNW can be grown laterally along Si(111) substrates that have been miscut toward [11− 2]. The ratio of lateral-to-vertical NW was found to increase as the miscut angle increased and as disilane pressure and substrate temperature decreased. Based on this observation, growth parameters were identified whereby all of the deposited Au seeds formed lateral NW. Furthermore, the nanofaceted substrate guided the growth via a mechanism that involved pinning of the trijunction at the liquid/solid interface of the growing nanowire.

Next, the growth of selenide heterostructures was explored. Specifically, molecular beam epitaxy was utilized to grow In2Se3 and Bi2Se3 films on h-BN, highly oriented pyrolytic graphite and Si(111) substrates. Growth optimizations of In2Se3 and Bi2Se3 films were carried out by systematically varying the growth parameters. While the growth of these films was demonstrated on h-BN and HOPG surface, the majority of the effort was focused on growth on Si(111). Atomically flat terraces that extended laterally for several hundred nm, which were separated by single quintuple layer high steps characterized surface of the best In2Se3 films grown on Si(111). These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3 .

The last part of this dissertation was focused on a recently initiated and ongoing study of graphene growth on liquid metal surfaces. The initial part of the study comprised a successful modification of an existing growth system to accommodate graphene synthesis and process development for reproducible graphene growth. Graphene was grown on Cu, Au and AuCu alloys at varioua conditions. Preliminary results showed triangular features on the liquid part of the Cu metal surface. For Au, and AuCu alloys, hexagonal features were noticed both on the solid and liquid parts.
ContributorsRathi, Somilkumar J (Author) / Drucker, Jeffery (Thesis advisor) / Smith, David (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2014
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Description
Fluctuation Electron Microscopy (FEM) has become an effective materials' structure characterization technique, capable of probing medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO has been exercised in numerous studies, FEM is not yet a quantitative technique. The holdup has been the discrepancy

Fluctuation Electron Microscopy (FEM) has become an effective materials' structure characterization technique, capable of probing medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO has been exercised in numerous studies, FEM is not yet a quantitative technique. The holdup has been the discrepancy between the computed kinematical variance and the experimental variance, which previously was attributed to source incoherence. Although high-brightness, high coherence, electron guns are now routinely available in modern electron microscopes, they have not eliminated this discrepancy between theory and experiment. The main objective of this thesis was to explore, and to reveal, the reasons behind this conundrum.

The study was started with an analysis of the speckle statistics of tilted dark-field TEM images obtained from an amorphous carbon sample, which confirmed that the structural ordering is sensitively detected by FEM. This analysis also revealed the inconsistency between predictions of the source incoherence model and the experimentally observed variance.

FEM of amorphous carbon, amorphous silicon and ultra nanocrystalline diamond samples was carried out in an attempt to explore the conundrum. Electron probe and sample parameters were varied to observe the scattering intensity variance behavior. Results were compared to models of probe incoherence, diffuse scattering, atom displacement damage, energy loss events and multiple scattering. Models of displacement decoherence matched the experimental results best.

Decoherence was also explored by an interferometric diffraction method using bilayer amorphous samples, and results are consistent with strong displacement decoherence in addition to temporal decoherence arising from the electron source energy spread and energy loss events in thick samples.

It is clear that decoherence plays an important role in the long-standing discrepancy between experimental FEM and its theoretical predictions.
ContributorsRezikyan, Aram (Author) / Treacy, Michael M.J. (Thesis advisor) / Smith, David J. (Committee member) / McCartney, Martha R. (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)
Created2015
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Description
Off-axis electron holography (EH) has been used to characterize electrostatic potential, active dopant concentrations and charge distribution in semiconductor nanostructures, including ZnO nanowires (NWs) and thin films, ZnTe thin films, Si NWs with axial p-n junctions, Si-Ge axial heterojunction NWs, and Ge/LixGe core/shell NW.

The mean inner potential (MIP) and inelastic

Off-axis electron holography (EH) has been used to characterize electrostatic potential, active dopant concentrations and charge distribution in semiconductor nanostructures, including ZnO nanowires (NWs) and thin films, ZnTe thin films, Si NWs with axial p-n junctions, Si-Ge axial heterojunction NWs, and Ge/LixGe core/shell NW.

The mean inner potential (MIP) and inelastic mean free path (IMFP) of ZnO NWs have been measured to be 15.3V±0.2V and 55±3nm, respectively, for 200keV electrons. These values were then used to characterize the thickness of a ZnO nano-sheet and gave consistent values. The MIP and IMFP for ZnTe thin films were measured to be 13.7±0.6V and 46±2nm, respectively, for 200keV electrons. A thin film expected to have a p-n junction was studied, but no signal due to the junction was observed. The importance of dynamical effects was systematically studied using Bloch wave simulations.

The built-in potentials in Si NWs across the doped p-n junction and the Schottky junction due to Au catalyst were measured to be 1.0±0.3V and 0.5±0.3V, respectively. Simulations indicated that the dopant concentrations were ~1019cm-3 for donors and ~1017 cm-3 for acceptors. The effects of positively charged Au catalyst, a possible n+-n--p junction transition region and possible surface charge, were also systematically studied using simulations.

Si-Ge heterojunction NWs were studied. Dopant concentrations were extracted by atom probe tomography. The built-in potential offset was measured to be 0.4±0.2V, with the Ge side lower. Comparisons with simulations indicated that Ga present in the Si region was only partially activated. In situ EH biasing experiments combined with simulations indicated the B dopant in Ge was mostly activated but not the P dopant in Si. I-V characteristic curves were measured and explained using simulations.

The Ge/LixGe core/shell structure was studied during lithiation. The MIP for LixGe decreased with time due to increased Li content. A model was proposed to explain the lower measured Ge potential, and the trapped electron density in Ge core was calculated to be 3×1018 electrons/cm3. The Li amount during lithiation was also calculated using MIP and volume ratio, indicating that it was lower than the fully lithiated phase.
ContributorsGan, Zhaofeng (Author) / Mccartney, Martha R (Thesis advisor) / Smith, David J. (Thesis advisor) / Drucker, Jeffery (Committee member) / Bennett, Peter A (Committee member) / Arizona State University (Publisher)
Created2015
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Description
This dissertation research has involved microscopic characterization of magnetic nanostructures using off-axis electron holography and Lorentz microscopy. The nanostructures investigated have included Co nanoparticles (NPs), Au/Fe/GaAs shell/core nanowires (NWs), carbon spirals with magnetic cores, magnetic nanopillars, Ni-Zn-Co spinel ferrite and CoFe/Pd multilayers. The studies have confirmed the capability of holography

This dissertation research has involved microscopic characterization of magnetic nanostructures using off-axis electron holography and Lorentz microscopy. The nanostructures investigated have included Co nanoparticles (NPs), Au/Fe/GaAs shell/core nanowires (NWs), carbon spirals with magnetic cores, magnetic nanopillars, Ni-Zn-Co spinel ferrite and CoFe/Pd multilayers. The studies have confirmed the capability of holography to describe the behavior of magnetic structures at the nanoscale.

The phase changes caused by the fringing fields of chains consisting of Co NPs were measured and calculated. The difference between chains with different numbers of Co NPs followed the trend indicated by calculations. Holography studies of Au/Fe/GaAs NWs grown on (110) GaAs substrates with rotationally non-uniform coating confirmed that Fe was present in the shell and that the shell behaved as a bar magnet. No fringing field was observed from NWs with cylindrical coating grown on (111)B GaAs substrates. The most likely explanation is that magnetic fields are confined within the shells and form closed loops. The multiple-magnetic-domain structure of iron carbide cores in carbon spirals was imaged using phase maps of the fringing fields. The strength and range of this fringing field was insufficient for manipulating the carbon spirals with an external applied magnetic field. No magnetism was revealed for CoPd/Fe/CoPd magnetic nanopillars. Degaussing and MFM scans ruled out the possibility that saturated magnetization and sample preparation had degraded the anisotropy, and the magnetism, respectively. The results suggested that these nanopillars were not suitable as candidates for prototypical bit information storage devices.

Observations of Ni-Zn-Co spinel ferrite thin films in plan-view geometry indicated a multigrain magnetic domain structure and the magnetic fields were oriented in-plane only with no preferred magnetization distribution. This domain structure helps explain this ferrite's high permeability at high resonance frequency, which is an unusual character.

Perpendicular magnetic anisotropy (PMA) of CoFe/Pd multilayers was revealed using holography. Detailed microscopic characterization showed structural factors such as layer waviness and interdiffusion that could contribute to degradation of the PMA. However, these factors are overwhelmed by the dominant effect of the CoFe layer thickness, and can be ignored when considering magnetic domain structure.
ContributorsZhang, Desai (Author) / Mccartney, Martha R (Thesis advisor) / Smith, David J. (Thesis advisor) / Crozier, Peter A. (Committee member) / Petusky, William T (Committee member) / Chamberlin, Ralph V (Committee member) / Arizona State University (Publisher)
Created2015